The case in front of me: a 25G drawing needle · 2 mg/mL.
I want the working, not the result — I need to be able to redo it with different numbers.
I care about the precision as well as the value — I want to know how many figures are real.
Is my approach right even if my number is wrong?
At 2 mg/mL every microlitre left behind is 2 µg, so a 50 µL hub costs 0.1 mg per draw and a 5 µL fixed-needle barrel costs 0.01 mg. Multiply by the draws, not by the doses: ten draws through a 50 µL dead space is 1 mg gone, which at 2 mg/mL is 0.5 mL of solution you paid for and never administered. Against a 2 mg dose that 50 µL is 5 per cent; against a 0.25 mg dose it is 40 per cent, which is why the loss matters most at exactly the doses where you can least afford it. a 25G drawing needle has a bore, a hub and a length, and the hub dominates: a fixed-needle insulin barrel has almost none, a luer connection has a measurable one before the needle even starts.
The single most important fact about dead space is that it is almost entirely in the hub cone, not in the needle, which is why changing needle gauge or length barely changes your losses.
At 5 mg/mL that is 15 to 25 µg lost per draw on the insulin syringe and 175 to 500 µg on the luer-lock — which over ten draws is the difference between losing a rounding error and losing half a milligram.
Specifically, delivered peptide = 10 x 0.5 mg = 5.0 mg. Lost to dead space = 10 x 84 µL = 840 µL x 0.005 = 4.2 mg. Yield = 50 per cent.
The World Health Organisation guidance on injection equipment adopted the same high-versus-low dead-space distinction, using a low-dead-space threshold in the low single-digit microlitres.
If cost matters, this is the first thing to change, not the last.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsWorth being precise here: at 100 µL draws the dead-space penalty with a luer-lock is 84 per cent per draw — the cost is genuinely catastrophic.
Draws available = 2000 / 102 = 19.6, so 19 full draws. Delivered peptide = 19 x 0.5 mg = 9.5 mg. Lost to dead space = 19 x 2 µL = 38 µL x 0.005 = 0.19 mg. Yield = 95 per cent.
The needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.
Syringe residual volume has been measured properly, mainly in the infection-control literature, with a median residual of about 84 µL for a conventional 1 mL syringe with a detachable needle and roughly 2 µL for a fixed-needle low-dead-space design.
Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.
The short answer is that dead space is small in absolute terms and huge as a fraction of a small dose, which is why it feels like a rounding error and behaves like a systematic loss.
The luer cone of the syringe plus the needle's own plastic hub accounts for the vast majority of the dead space.
Corollary that follows immediately: changing needle gauge or length barely changes your losses.
The switch nearly doubles your vial, which is better than most other optimisations combined.
It helps to be literal here: a fixed-needle insulin syringe holds roughly 3 to 5 µL dead space and costs the same as a luer-lock syringe with 35 to 100 µL dead space.
Configuration B — 0.5 mL fixed-needle U-100 insulin syringe, dead space 2 µL: volume removed per draw = 100 + 2 = 102 µL.
Published inter-laboratory comparisons of dead-space measurements on identical syringes show good agreement, suggesting the numbers are reliable.
One qualification: the dead space does not affect the dose accuracy if the hub was full of solution at the start of the draw.
Write the arithmetic on the vial label. It costs nothing and it removes the step where you reconstruct it from memory at an inconvenient moment.
edited 28 Jan 2025 by u100_marks — updated for the 2026 guidance change
In practice, the switch to a low-dead-space syringe nearly doubles your usable vial, which is better than switching suppliers if you are looking for cost savings.
Be sceptical of anything advertised as low dead space that retains a conventional plunger tip: if you can look into the fitting with the plunger fully forward and see an open conical void, that void is your dead space.
Worth noting: draw size matters enormously — the smaller your draws, the more the syringe architecture matters.
The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale against the barrel rather than against your assumption.
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